Devices of this description are described and claimed in my prior U.S. Pat. Nos. 3,775,517 and 4,379,277. The earlier one of these patents discloses a face plate consisting of alternating pole pieces and nonmagnetic spacers, that plate being supported by a frame in which a core is movable between the aforementioned positions. The core and the frame include bar magnets and additional pole pieces whose relative alignment or disalignment forms circuits for the magnetic flux which either are closed in the device itself or pass through workpieces to be attracted. My second patent shows a device of the same general type including a longitudinally shiftable stack of flat permanent magnets and intervening flat pole pieces which are selectively alignable with other, fixedly positioned pole pieces that are polarized by further, stationary permanent magnets. Another device of this character is the subject matter of my copending applications Ser. No. 478,078 filed Mar. 23, 1983, now U.S. Pat. No. 4,465,993, that device comprising a permanently magnetized rotor bracketed by stationary pole pieces.
Reference may additionally be made to my prior U.S. Pat. No. 4,075,589 disclosing a holder rendered operative by a set of electromagnets.
The frame forming part of the device of my first-mentioned patent leaves only the face plate exposed for the alternate retention and release of a workpiece. U.S. Pat. No. 4,379,277 shows a structure which also exposes lateral edges of the stationary pole pieces whereby sides of the chuck form ancillary workpiece-retaining surfaces.